One of the common difficulties in an optical information recording disk, e.g., laser disk, lies in the occurrence of focussing errors. An astigmatic method has been introduced to solve the problem.
In FIG. 1, there is illustrated a prior art optical system 10 utilizing the astigmatic method, as disclosed in U.S. Pat. No. 4,023,033, entitled "Optical Focussing Device" and is incorporated herein by reference.
The optical pickup system 10 comprises a light source 11, a beam splitter 12, an objective lens 13, an optical information recording disk 14 (hereinafter referred to as an optical disk), a cylindrical lens 15, a first adder 16, a second adder 17, an optical detector 18 and a differential amplifier 19. In the system, a light beam 20 emitted from the light source 11, e.g., a laser diode, enters the beam splitter 12 and is partially reflected by a reflection surface 21 incorporated therein. The light beam reflected from the reflection surface 21 is radiated through the objective lens 13 onto a recording surface 22 of the optical disk 14 as a focussed light beam. The focussed light beam reflected from the optical disk 14 is converged by the objective lens 13 and transmitted through the beam splitter 12. The focussed light beam transmitted through the beam splitter 12 is made astigmatic by its passage through the cylindrical lens 15 and thereafter is made to impinge onto the optical detector 18 including a square light-reception surface 23 formed by arranging four square photoelectric cells (not shown). Each of the photoelectric cells generates an output in the form of a light intensity measurement. Two outputs from two opposite corners of the square light-reception surface 23 are sent to the first adder 16, and two outputs from the remaining two opposite corners are sent to the second adder 17, respectively. Outputs from the first and second adders 16, 17 are then sent to the differential amplifier 19, which will in turn generate an associated focussing error signal by comparing the outputs from the first and second adders 16, 17, the focussing error simply being a difference of the two outputs therefrom. Being astigmatic, the shape of the luminous flux imaged on the four square photoelectric cells of the square light-reception surface 23 of the optical detector 18 changes depending on the positional relationship between the recording surface 22 of the optical disk 14 and a convergence point 24. In order to detect this change in the shape of the luminous flux, the cylindrical lens 15 is arranged exactly between the convergence point 24 and the optical detector 18 in such a way that the square light-reception surface 23 is disposed at the position where the shape of the luminous flux becomes circular when it is focussed (zero focussing error) and this is known a "just focussed" position. If the optical disk 14 is displaced in a vertical direction from the just focussed position, the focussing error signal generated by the differential amplifier 19 becomes non-zero with the sign indicating the direction of displacement. This conventional astigmatic method requires the cylindrical lens 15 to focus the light beam spot on the square light-reception surface 23 in an astigmatic manner. Since, however, the conventional cylindrical lens employed therein is not planar, it is rather difficult to align it accurately with the convergence point 24 and the optical detector 18.